The present invention relates to an asymmetric porous polytetrafluoroethylene membrane for clothing and a material for clothing using the membrane as an air permeable and waterproof membrane.
Porous polytetrafluoroethylene membrane (hereinafter referred to as porous PTFE membrane) has excellent chemical resistance and high tensile strength and therefore is suitably used for a variety of purposes such as a filter for filtering gas and liquid, an agent for an air permeable and water impermeable membrane for clothing and a sheet for medical use, in addition to a sealing or gasket for piping and production facilities in the fields of chemicals, foods and semiconductors.
A process for preparing a porous PTFE membrane is disclosed (for example see U.S. Pat. No. 3,953,566, U.S. Pat. No. 3,962,153, U.S. Pat. No. 4,096,227, U.S. Pat. No. 4,187,390 and U.S. Pat. No. 4,902,423), in which PTFE paste, which is a mixture of PTFE fine powder and an extrusion aid such as naphtha, is extruded and then rolled. Then, after removing the extrusion aid from the rolled article, the article is drawn in a uniaxial or biaxial direction. Subsequently, to maintain the shape of the drawn porous PTFE membrane, heat-setting is conducted at a temperature between 35° C. and the melting point of PTFE.
However, the porous PTFE membrane obtained by the above method has excellent air permeability as a laminating agent for clothing material but there is the problem that lipid from the human body accumulates in the porous membrane, decreasing the air permeability properties.
In order to solve the above problem, disclosed is a material for clothing obtained by treating a porous PTFE membrane with a hydrophilic and air permeable material such as polyurethane (for example see U.S. Pat. No. 4,194,041). However, there are problems such as water vapor permeability is low and cleaning resistance is low.
Also, a method for laminating a porous PTFE membrane and fabric such as polyester and nylon is disclosed (for example see U.S. Pat. No. 5,026,591). However, adhesion strength between the materials is unsatisfactory and the problem of durability as material for clothing remains.
A material for clothing having satisfactory water resistance, air permeability, water vapor permeability and adhesion strength cannot be obtained with the symmetric porous PTFE membrane described above.